The present invention relates generally to metal layers deposited on rigid substrates, the process of their deposition, the substrate formed therefrom, and the utility of such substrates. In particular, the present invention is directed to processes for depositing one or more metal layers, at least one of which based on tungsten-containing or molybdenum-containing precursors, onto rigid substrates by a variety of methods.
The present invention relates to rigid substrates, especially transparent or semi-transparent ceramic-type substrates, such as glass or glass-ceramic substrates or substrates that are barely transparent or non-transparent, and therefore relates to substrates that are generally insulating from an electrical standpoint and that are capable of withstanding heat treatments without deterioration.
Although the present invention is not limited to such an application, as other applications will be detailed within, the present invention are first described with reference to the production of electronic devices, especially to the production of emissive screens, more particularly to those called xe2x80x9cflatxe2x80x9d screens of the plasma-screen type.
A plasma screen essentially consists of two glass substrates constituting the front and rear faces of the screen, on which substrates functional layers have been deposited. Those layers, deposited on the opposing faces of the substrates, comprise, especially, electrodes, at least a coating of a dielectric material and phosphors, these elements being indispensable for the operation of the screen.
The present invention thus concerns the manufacture of substrates provided with electrically conducting layers capable of forming electrodes after suitable etching treatments, most particularly those using layers made of highly conducting metals, such as silver and copper.
It is known that copper and silver layers may be deposited by electrolysis, which is an attractive technique since it is possible to obtain relatively thick metal layers (for example a thickness of between 1 and 5 xcexcm) that can be necessary to achieve the required level of conductivity in plasma screens. However, electrolysis can only be implemented in a reasonable production time if the substrate to be covered is itself sufficiently conducting at the start. In general, at least for the front faces of plasma screens, provision is made to interpose a transparent electrically conducting layer made of a doped metal oxide between the glass substrate and the metal electrode layer. When one or more of these layers is etched, the doped metal oxide layer may optionally be etched in a pattern different from that of the metal electrode. This transparent layer may be made of, for example, fluorine-doped tin oxide, F:SnO2, or of tin-doped indium oxide, Sn:In2O3. The thicknesses in which the doped metal oxide layer is used are chosen for maintaining its transparency, but proved not to be conductive enough to allow electrolysis under satisfactory conditions.
The impetus for the present invention was therefore to remedy this drawback, especially by developing a novel conducting layer which is especially able to serve for xe2x80x9cpreparingxe2x80x9d the surface of an insulating substrate of the glass, ceramic, or glass-ceramic type, in order to allow another metal conducting layer to be deposited by electrolysis.
The present invention is directed to a process for depositing at least one metal layer based on tungsten or molybdenum by chemical vapor deposition on a non-conductive substrate, which comprises providing at least one tungsten- or molybdenum-containing precursor in the form of a metal halide or an organometallic compound and at least one reducing agent, such as silane or hydrogen, to form the at least one metal layer.
The metal halide is preferably a metal chloride, such as WCl6 or MoCl5, or a metal fluoride, such as MoF5.
The organometallic compound may especially be a metal carbonyl, such as W(CO)6, or alkyl-tungsten or allyl-tungsten compounds, such as butadiene-tungsten or bio-benzene tungsten.
Tungsten and molybdenum have a number of advantages when used in the present invention. They are both good electrical conductors, although less conducting than copper or silver. Therefore, these metals may form an integral part of an electrode, consisting essentially of the layer according to the present invention, and/or an integral part of an optional second conducting layer of the copper or silver type, thus contributing to a possible increase in the electrical conductivity that a copper or silver layer alone may have.
Furthermore, these W-based or Mo-based layers lend themselves well to pyrolytic deposition in the vapor phase (also called chemical vapor deposition or CVD) at atmospheric pressure, especially as metal halides, the decomposition temperature of which is typically compatible with the substrates envisaged in the present invention, especially of the glass type. Deposition by CVD may advantageously result in layers which generally adhere well to the substrate and are typically very durable.
Indeed, these W-based or Mo-based layers have proven to be particularly compatible with the materials with which they may come into contact: they coat out well, especially on a surface made of glass and of a metal oxide of the F:SnO2 or Sn:In2O3 type. Furthermore, molybdenum and tungsten are metals quite similar to copper or silver, particularly in crystal lattice structure. The W and Mo layers also offer nucleation sites for subsequently deposited copper or silver layers, which are deposited very satisfactorily on their surface, to which they tend to adhere very strongly.
Additionally, tungsten and molybdenum are metals which are resistant to solutions usually employed for depositing copper electrolytically, these solutions generally being very highly acidic (pH around 1). Unexpectedly, the W-based or Mo-based layers may however be etched, if necessary, using known chemical etching techniques, especially using oxidizing solutions containing aqua regia and hydrofluoric acid.
Advantageously, when the envisaged substrate is made of glass, a W-based or Mo-based layer may be deposited directly on the ribbon of glass on a float line, especially in the floating chamber. The WCl6-type precursors thus decompose at around 600 to 630xc2x0 C., and at lower temperatures in the case of WF6, temperatures at which the ribbon of glass has generally acquired, on the float line, its dimensional stability. The atmosphere of the float bath is favorable for deposition in that it is slightly reducing.
Another advantage of molybdenum and tungsten is that they are not very oxidizable, especially at relatively high temperatures. Thus, if the layers are deposited on the ribbon of glass in the floating chamber, they generally do not exhibit little or no oxidizing while the ribbon of glass passes through the lehr.
According to a first embodiment, the W-based or Mo-based layer according to the present invention may be deposited directly on its carrier substrate, especially directly on the glass.
According to a second variant, the W-based or Mo-based layer may be deposited on a layer deposited beforehand on the substrate. There may be one or more pre-deposited layers on the substrate before the deposition of the W-based or Mo-based layer. As mentioned above, this may be a transparent conducting layer of doped metal oxide type, for example, such as fluorine-doped or antimony-doped tin oxide, aluminum-doped zinc oxide, or tin-doped indium oxide. These layers may also be deposited, in a known manner, by chemical vapor deposition or by pyrolysis in the pulverulent phase (reference may be made, for example, to patents EP 158,399, EP 357,263 and WO 88/00588 for the deposition of F:SnO2 layers by CVD and to patents EP 573,325 and WO 94/25410 for the deposition of F:SnO2 layers by pyrolysis of powder). Two successive layers may thus be deposited pyrolytically on the float line when the substrate is a glass substrate. Of course, it is possible to interpose or to add another type of layer providing another functionality and/or contributing to the electrical conductivity of the metal layers. These other layers may thus act as xe2x80x9cbarrierxe2x80x9d layers (for example, to the alkali metals of the glass), such as layers based on SiO2 or on a silicon oxycarbide or oxynitride, such as those described, for example, in patent EP 518,755, and which layers may be deposited beforehand on the ribbon of float glass by chemical vapor deposition.
As mentioned above, provision may advantageously be made to at least partially cover the W-based or Mo-based layer according to the present invention with a layer made of a metal or a metal alloy differing from tungsten or molybdenum, especially one that is more conducting than these two metals, such as copper or silver. This layer may be satisfactorily deposited by electrolysis, for example, in a subsequent step, up to a thickness of several microns, in part due to the level of electrical conductivity, the presence of nucleation sites, and the good adhesion conferred on the substrate by the W-based or Mo-based layer(s). The electrolytic deposition may be carried out in DC mode or in pulsed-current mode.
In pulsed-current electrolytic deposition of non-W-based or non-Mo-based layers, it is preferable to cover-this, for example, copper or silver layer with its own protective layer, especially one which inhibits oxidative degradation. The subsequent deposition of a layer of, for example, a low-melting-point enamel used for the manufacture of plasma screens, may afford such protection. The preferred protective layer is relatively thin and includes metal, especially at least one metal chosen from Ni, Cr, Ti, Ta, Nb, Pd, and Zr, more especially an NiCr alloy.
A metal layer is preferable since it ensures that there is electrical contact with the subsequent layers. It may be deposited in a known matter, for example, by sputtering, especially magnetron sputtering, using a suitable target of metal, or by electrolysis like the layer that it protects.
The subject of the present invention is also directed to a process for electrolytically depositing a metal layer based on copper and/or silver on a glass, ceramic or glass-ceramic substrate, optionally already covered, especially with a transparent electrically conductive layer of a doped metal oxide, especially of the type of those mentioned above. The electrolysis may be carried out by making the surface of the substrate to be covered sufficiently conducting, for example, by pre-depositing, for example, by chemical vapor deposition, a metal layer containing at least tungsten or molybdenum metals upon reacting a precursor of one or more metals with a reducing agent, for example, such as those mentioned above.
The W-based or Mo-based layer of the present invention thus makes it possible to substantially utilize advantages of the electrolysis process, which is very suitable for depositing layers on the order of microns thick. The electrolytically deposited layers, furthermore, generally contain few, if any, pinhole defects, which are typically encountered in vacuum deposition techniques of the sputtering type. The cost of an electrolytic process is also much lower than that of a vacuum deposition process. Furthermore, by suitable selective masking of a substrate before immersing it in an electrolysis bath, one or more layers in a desired pattern may be obtained directly, without an etching step separate from the deposition being necessary.
After deposition, etching may be performed on at least one of the aforementioned layers, especially to aid in the formation of electrodes, in defined patterns suitable for the desired applications.
The present invention is also directed to a rigid transparent substrate, for example, such as glass, ceramic, or a glass-ceramic composite, which substrate can be provided with layers, including a metal layer based on tungsten and/or molybdenum, especially deposited by chemical vapor deposition, and including a metal layer based on copper and/or silver, especially deposited by electrolysis. These substrates may advantageously be obtained by any of the processes described above.
It is also within the scope of the present invention to deposit the W-based or Mo-based layer by deposition techniques other than pyrolysis or CVD. These may especially be vacuum deposition techniques of the sputtering type, optionally magnetron sputtering. Particularly with magnetron sputtering, the substrate is generally organic, as opposed to previously stated inorganic substrates, and can include one or more polymers made of, for example, rigid transparent material, such as polymethyl methacrylate (PMMA) or polycarbonate.
As mentioned above, the substrate may be provided with one or more other functional layers, especially beneath the W-based or Mo-based metal layer. The one or more other layers may include, for example, a transparent electrically conducting layer of the doped- metal oxide type, as mentioned above. In the production technique for manufacturing emissive screens, especially, in this case, on a xe2x80x9cfrontxe2x80x9d face beneath metal electrodes, which are preferably based on copper and/or silver, it may be preferable to provide another series of electrodes, obtained by etching but based on transparent conducting layers. The presence of the other series of electrodes can help reduce the strike voltage between two facing electrodes of the front and rear faces of the emissive screen, as well as to make a plasma discharge between two glass panels thereof homogeneous. The electrically conducting layer may be deposited pyrolytically or by sputtering, especially reactive sputtering in an oxidizing atmosphere, using a target made of an ad hoc metal alloy. The thickness of said layer is preferably from 50 nm to 500 nm, preferably between 100 nm and 400 nm in the case of F:SnO2, and preferably between 50 nm and 120 nm in the case of Sn:In2O3.
The silver-based or copper-based layer may optionally be further coated with a protective layer based on at least one of the metals belonging to the group comprising chromium, nickel, titanium, tantalum, niobium, palladium, and zirconium, preferably based on a NiCr alloy, a steel, or a chromium alloy. The thickness of the further protective layer is preferably sufficient to fulfill its protective role without unnecessary or significant detrimental effects to cost and/or deposition time, more preferably from 10 nm to 300 mn, most preferably between 30 nm and 200 nm.
Preferably, the thickness of the W-based or Mo-based layer is determined by the level of conductivity required for carrying out the electrolysis, which level may especially be evaluated as a resistivity of 8xc3x9710xe2x88x925 ohm*cm, corresponding to a surface resistance less than or equal to 10 ohms per square. Preferably, the thickness is from 30 nm to 500 nm, more preferably between 50 nm and 300 nm, most preferably between 70 nm and 130 nm.
With respect to the copper or silver layer, the thickness is preferably between 0.1 xcexcm and 10 xcexcm, more preferably between 0.5 xcexcm and 5 xcexcm, most preferably between 1.5 xcexcm and 3 xcexcm.
The applications of the substrates described above may vary greatly and therefore may involve a number of further processing steps. Thus, if a glass-type substrate is intended to be used as a front face of an emissive screen, it may be advantageously arranged to cover the substrate, once the etching operation(s) has (have) been completed, with a protective coating including at least one layer of a dielectric material, especially one having a low melting point, preferably of the PbO type, or of a mixture of various oxides. Alternately, if a glass-type substrate is intended to be used as the rear face of an emissive screen, the substrate provided with its etched layers can be subsequently covered, at least partially, preferably with one or more coatings, of materials containing, for example phosphors.
The substrate according to the present invention, if intended to be used as the rear face of an emissive screen, can preferably be xe2x80x9ccompartmentalizedxe2x80x9d into pixels separated from one another by walls generally made of glass having a low melting point, for example, a PbO-based glass.
The tungsten or molybdenum layer according to the present invention may also be used alone on a substrate to functionalize it. For example, on a glass-type transparent substrate, this sole layer may give impart reflective properties in the thermal radiation range, for example, as a thermal protection glazing, or in the visible range, for example, as a glazing with a moderate level of light transmission, e.g., for buildings or vehicles. Alternately, the present invention may serve as a mirror. Similar applications may also be envisioned if the present invention is further coated with a copper or silver layer and optionally with one or more protective layers.
The tungsten or molybdenum layer may more generally be employed wherever electrodes or current leads are required, again optionally combined with a copper or silver layer. Thus, the present invention may be used as resistance heating elements, for example, for anti-fogging/anti-frosting heated widows, glass-ceramic plates used in cookers, or even for forming heating mirrors. Of course, the thickness of the W-based or Mo-based layer may be adapted to the application, as can other properties of the substrate, the present invention, or any layer thereof, to suit the particular application.